It is well known that there is a large body of literature dealing with the preparation of silica sols wherein the silica particles have various diameters and the sols are prepared by many different methods. A list of patents and literature is given on pages 91-100 of Surface and Colloid Science published in 1973 by John Wiley & Sons. Among other things, this publication discloses on pages 12-15 methods of increasing the particle size of the silica particles in silica sols by heating. In one such process disclosed by Rule in U.S. Pat. No. 3,012,972, a sol consisting of particles from 8-15 millimicrons (m.mu.) in diameter is first thoroughly deionized, and then autoclaved at from 300.degree.-350.degree. C. under autogenous pressures. This is said to produce silica particles in the diameter range from 88 to 150 m.mu. which can be concentrated to a stable sol containing over 60% by weight of silica. This process involves the use of an acidic pH during the heating step and an acidic pH tends to produce gels. It also requires the use of deionized water in making the starting sol. The concentration of the sol is effected by boiling or otherwise removing water from the sol produced by the process.
It is known to prepare silica sols containing a majority of large particles in separate steps using starting sols in which the particles are substantially uniform or have a relatively narrow range of size distribution. For example, a silica sol having an average silica particle diameter of 20 millimicrons can be prepared by heating a silica sol having an average silica particle diameter of 15 millimicrons. A silica sol having an average silica particle diameter of 40 millimicrons can be prepared by heating a silica sol having an average silica particle diameter of 20 millimicrons. A silica sol having an average silica particle diameter of 60 millimicrons can be prepared by heating a silica sol having an average silica particle size diameter of 40 millimicrons. However, this preparation of silica sols containing silica particles having an average diameter of 60 millimicrons requires three separate distillations. Moreover, in this type of preparation the particle formation occurs primarily by the same sized particles condensing with one another.
It would be desirable to prepare silica sols containing large sized silica particles in a single step and this is the primary object of the present invention.
Another object of the invention is to provide a method of preparing silica sols containing silica particles of larger sizes which have not heretofore been available and have not been prepared by conventional methods, and especially new and improved silica sols in mono-dispersed form wherein the majority, if not practically all of the silica particles have large diameters so that the resultant sols are especially useful as abrasive materials in mechanically polishing semi-conductor wafers such as silicon, sapphire, and gadolinium-gallium-garnets (GGG wafers).